Survey Methods

Field Methods

Minimum criteria for YOY American Eel sampling were established in the Atlantic States Marine Fisheries Commission American Eel Fishery Management Plan (pdf) and include the following:

  •  The timing and placement of gear must coincide with periods of peak YOY shoreward migration.
  • At a minimum, the gear must fish during flood tides during nighttime hours.
  • The sampling season is designated as a minimum of 4 days per week for at least 6 weeks, or for the duration of the run.
  • At least 1 site must be sampled in each jurisdiction.
  • The entire catch of YOY eels must be counted from each sampling event and a minimum of 60 glass eels (if present per system) must be examined for length, weight, and pigmentation stage weekly.
The VIMS American Eel Monitoring Program has sampling sites on the James, York, and Rappahannock rivers (shown here), and also on the Potomac River. Click image for larger version.Sampling Sites

Each year, six sites are sampled from approximately February to June:

  • Bracken's and Wormley Pond on the York River
  • Wareham's Pond on the James River
  • Kamp's Millpond on the Rappahannock River
  • Gardy's and Clark's Millpond* on the Potomac River    
*Sampling at Clark's Millpond was discontinued after 2016.

Sampling Gear

Irish eel ramps (below) are used to collect eels at all sites. Ramp operation requires a continuous flow of water over the climbing substrate and into the collection device and is accomplished through a gravity feed. EnkamatTM erosion control material on the ramp floor provides a textured climbing surface. The ramps are placed on an incline (15-45°) with the ramp entrance and textured mat extending into the water. The ramp position, along with the additional 4° incline of the substrate inside the ramp, provides sufficient slope to create attractant flow. 

irish eel ramp

Only eels in the ramp's collection bucket (not on the climbing surface) are recorded. Trap performance is rated on a scale of 0 to 3 (0 = new set; 1 = gear fishing; 2 = gear fishing, but not efficiently; 3 = gear not fishing) and water temperature, air temperature, and precipitation are recorded during most site visits. All eels are enumerated and placed above the impediment, with any subsample information recorded, if applicable. Specimens less than or equal to ~ 85 mm total length (TL) are classified as YOY, while those > 85 are considered elvers. Length, weight, and pigmentation stage are recorded from 60 eels weekly.

Index Calculations 

A review of the index calculation procedure was undertaken in 2009 to investigate the use of the geometric mean catch for days during which 95% of the glass eels were captured. Three recruitment indices were calculated and compared:

  1. the simple, arithmetic average over the time period sampled,
  2. the geometric mean using the 95% cut-off, and
  3. the area-under-the-curve (AUC).

 The AUC method was determined to be the most appropriate for the survey, as is not sensitive to differences in annual sampling effort that may result in additional days with zero catches. More importantly, the index can easily accommodate variations in daily recruitment patterns that may be environmentally driven and vary from year to year.